Tell CMake where to find ICU properly

I had a problem where CMake picked up half of ICU from its C API, which is included with the Windows SDK, and half from the C++ API we provide. This should prevent that.

• ICU_ROOT takes precedence as a CMake variable so do that instead of as an environment variable.
• ICU_LIBRARY is an output of FindICU.cmake, not an input, so don't set it.
• FindICU.cmake needs telling about components via their own variables.